"Maximum average number or biomass of organisms that can be sustained in a habitat over the long term. Usually refers to a particular species, but can be applied to more than one. " Hyperdictionary definition.

For other definitions explore the web. Here is one link.

How much fish biomass can you keep alive in a fish tank, or in a pond?

Diagram indicating how carrying capacity is not constant with time, but changing.

The carrying capacity can be exceed for some time, but this is associated with a decrease in the carrying capacity. It is conceivable that the system can regenerate its carrying capacity, which introduces the concept of a recovery time. Hidden in all of this is an argument about whether natural systems spend much time at any equilbrium state, or whether most systems are in perpetual non-equilbrium.

One can think also of local versus global carrying capacities. Trade and commerce make definition of a local carrying capacity for people living in some area much more difficult to ascertain. The concept of ecologic footprint has been developed out of this. This is the area and associated resources needed to support a given population in an area. As you might guess estimates of ecologic footprints indicate that industrialized countries have footprints several times their actual area, while less 'developed' countries have much smaller footprints. However, it should be stressed that this is a type of analysis in its infancy.

Examples where there has been a local change in carrying capacity:

• Cyprus and the pygmy hippos.
• Pygmy mammoths on Catalina Island.
• Vikings on Greenland.
• Akkadian empire: in Tigres-Euphrates area: sudden collapse at 4170+/-150 yr B.P.Coincides with a sudden (with a sudden aridity episode 300 years long). Local climatic shift. Cullen et al., 2000, Climate change and the collapse of the Akkadian empire: Evidence from the deep sea; Geology, vol. 28, p. 379-382.

Florida Keys Carrying Capacity Study.

Bongaarts (1994) - perhaps possible to feed 10 billion people in 2050. But if doubling time 50 years how about in 2100?

Now we can progress from focusing on just one population of interest and start to think of how two codependent populations can interact. This is classic biology.

Snow-shoe rabbit and lynx linked cyclic variation - cycles interpreted as due to internal dynamics.

You have one equation that describes how the rabbit population is dependent on lynxes and another that describes how lynxes are dependent on rabbits and you operate them simultaneously. This is an example of a negative feedback loop.

This type of prey-predator relationship can be modeled with Stella software. The boxes represent the rabbit and fox populations with inflows (increases) by birth and outflows (decreases) by death rate. The circles represent factors that influence an inflow or outflow. Very simple equations determine the inflows and outflows. Note how the arrows indicate that the rabbit death rate is a function of the fox population and fox birth rate is a function of the number of rabbits available. The two are linked. Stella then mathematical models the relationship. and produces the model history of populations seen. Change some of the parameters and the specific history changes also. However, the general behavior of linked, repetitive spikes is a robust result. Compare the general form of the model with the actual history shown in lecture for lynxes and rabbits.

While this is an often utilized success story, a good question is how widely applicable is it. Do most biologic populations show such cyclic behavior? What happens when you have three or more populations in prey-predator relationships? Stella allows modeling of this also.

Alexei Sharov, VPI Population ecology web resource site.

When I was a boy we had an outbreak of Japanese beetles. To save our rose bushes and other plants my father gave us kids a small jar half full of gasoline and we were given five cents for every hundred beetles we flicked from the plants into that jar. We could fill jars with beetles packed in gasoline with ease. I also spent time down in South Carolina as a student, and the stories about kudzoo down there are legendary. Using the key words Zebra Mussels and Great Lakes and you will find abundant information on yet another invasive species.

Some estimates of the magnitude of concern (Pimental at al.):

• some 50,000 species introduced into the U.S..
• an estimate of $138 billion damages and losses per year.
• such species play a role in 42% of organisms on the threatened or endangered species list.

Definition for exotic and invasive species: These are organisms introduced by human action, purposeful or unintentionally, into an area they did not exist within before. They can be very disruptive to the local ecosystem. This is partly because they have no natural predators to provide a check and balance system, resulting in explosive population growth. There are now some stiff penalties (but perhaps not stiff enough) for those introducing exotic or invasive species.

Links to sites for further exploration:

• Site listing exotic-invasive species.
• University of Cornell article by Pimental et al. describing economic costs of invasive species.

Lemmings:

• Site with short description of lemmings.
• Lemmings have highs 1000 times the lows in their cyclic population swings.
• Lemmings suicidal or just short-sighted?

Algal blooms and the power of microbes:

• 

  ---

  Photo from USGS site of one of the microbes that causes 'red tide.' Alexandrium tamarense

  ---

• pond scum and water eutrophication.
• Biologic oxygen demand - BOD.
• Red tide - series of microbes that excrete toxins. Also known as HAB = Harmful Algal Bloom.
• HABs are on the increase. Why?
• Some links to explore if you have more time and interest.
  • University of Hawaii site on red tide.
  • Florida fisheries site on red tide.
  • USGS study - Toxic "Red Tide" Populations in the Western Gulf of Maine: Sources, Transport, and Nutrient Environment. The final report has maps documenting the map extent of red tide development.
  • American Geophysical Union summary of red tide and associated environmental issues by Don Anderson.